Ground noise elimination



Aug. 21, 1934.

C10. GREENTREE GROUND NOISE ELIMINATION Filed April 12. 1930 RECTIFIERDUUUDUUU Fig.5.

Invent o1":

Greentvee,

His Attorneg.

Patented Aug. 21, 1934 PATENT orrics 1,971,181 (mom NOISE umrmn'noxcrimes 1). Greentree, Schenectady, N. r., aaelgnor to General ElectricCompany; a corporation or New York Application April 12, 1930, SerialNo. 443,875

1 Claims. (Cl. ire-no.3)

My invention relates to the making of photographic sound records whichare adapted for use in the reproduction of the recorded sound. In thereproduction oi sound from a photographic film record as heretoforemade, whether the record be of the variable density type or the variablewidth type, there is usually in addition to the recorded sound a certainamount of so called ground noise". Such noise being of a substantiallyconstant intensity of volume is not noticeable as long as the recordedsound'is of the usual volume but whenever the sound volume reaches a lowvalue the ground noise in comparison becomes the more prominent. Groundnoise is particularly noticeable, lor example, during very soft passagesin musical reproductions and also before the recorded sound comes on andafter it ceases. It is known that a considerable part of the groundnoise heard when a film record is reproduced is a result of striations,scratches and dirt on the relatively clear portions of the record wherethe light passes freely into the photo-electric cell. Variousarrangements have been proposed for reducing the ground noise irom filmsound records by the use of supplementary apparatus applied to the soundrecorder for modifying the record in the original recording. In certaincases it is found to be objectionable to add anything to the recorderwhich will add oomplication thereto or require any additional attentionby the operator, this being particularly true in the case oi portableinstruments. It is the object of my invention to provide an improvedmethod and apparatus ior reducing, not eliminating, the cause of theobjectionable ground noise in a copy of the sound record.-

My invention will be better understood iroin the following descriptiontaken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

Referring to the drawing, Fig. l is a diagrammatic view of one term ofapparatus embodying my invention; Figs. 2 and 3 are detail views thereofand Figs. c and 5 are enlarged views of fragments respectively or anoriginal sound record iilm and a copy 'thereoi'msde in accordauce withmy invention.

In illustrating my invention I have shown apparatus ior making thephotographic copy lrom a previously made photographic film record whichrecord may be an original sound record or a duplicate thereof suchd'orexample as that made by printing or by ire-recording 1mm 1 him, aphonographic or a magnetic rec- 0rd. Moreover the sound record to becopied may be of either the variable density type or the variable widthtype.

In Fig.1 I have shown the film '1 being unwound from the roll 2 andbeing wound up on the roll 3, which film for example, may be an originalnegative soundrecord film, a fragment of which appears in Fig. 4. At 4is shown the sensitive film being unwound'irom the roll 5 and wound upon the roll 6 upon which film u the sound record on film 1 is beingprinted. The particular form of printing apparatus employed forms nopart of and is immaterial to my present invention. Various well knownfilm. printers may be used. For convenience of illustration I have shownin a diagrammatic manner a printer of the contact type wherein the twofilms 1 and 4 pass in contact beneath the printing light. In thisprinter. the two films are shown moved by engagement with the commonsprocket wheel 8 while p sing between members 9 and 10. Light from thesource 11 passes through the opening 12 'to the films. The two films areshown led to the printer by a common sprocket 14 and are withdrawn fromthe printer by the sprockets 15 and 16, the

driving means for the sprockets being omitted from the drawing. Betweensprocket blend the printer suitable friction rollers 17 are shown bymeans oi which the films are given the 35 proper tension upon enteringthe printer.

Between the film roll 2 and the sprocket 14 film 1 passes a gate shownfor example as the curved member 19 having the opening 20 arrangedopposite the sound record track on the At one side of the gate is thelight source and optical system 21 by means of which a narrow light beamis projected through opening so on the ills. the beam forming a line oflight men the him, see Fig. 2. Behind the film is the photo-electriccell 24 in a position to receive the light beam as modified by the soundrecord. Connected with the cell 24 is the amplifier 25,

the lull wave rectifier 26 and the filter :l'l i i-om there is receiveda direct my the latter of which current which varies substantially withthe am plitude of the sound waves recorded on film 1. Between film rollliand sprocket 14, film e passes a gate whichis shown for example as thecurved member 29 having therein the opening 30 arranged opposite thesound track oi film 4. By means of the light source-and optiaystem 31,which for example may or may not be similar to thatnhown at 21, a lightbeam uprolectedthrough the openinzwontctbe no sound track 01 him 4, Thelight thus projected on the film is controlled to give a supplementalexposure of the sound track in accordance with the output of the filter27 whereby said exposure shall be a minimum when the amplitude of thewaves recorded on film 1 have a given maximum value and shall be amaximum when the amplitude of the recorded waves are a minimum. Variousmeans may be employed thus to control the supplemental exposure of thesound track. In the form of my invention illustrated, the sound recordbeing printed is of the variable width type and the amount ofsupplemental exposure is controlled by the use of a shutter arranged inthe path of the light beam and movable transversely of the film inaccordance with the output of the filter 27. Referring to Figs. 1 and 3,the shutter 33 is mounted on the arm 34 attached to the coil 35 which isconnected with the filter 27 through switch 28. Coil 35 is mounted onpivots 36 within the field of a magnet having pole pieces 37. The smallstop pin 38 projecting from the member 29 is posltioned in the path ofmovement of the arm 34 which is yieldinglv held agaimt the stop by thecoil spring 39. In this position ofthe shutter substantially one half ofthe sound track is exposed, or ii preferred more than one half may beexposed. Energization of the coil 35 effects a counter-clockwiserotation oi the shutter as viewed in Fig. 3 whereby more and more lightis cut 0!! depending upon the degree of rotation. In Fig. 4, thenegative film 1 has the sound record 40 which in the region indicated by41 has a very large amplitude and in the region indicated by 42 has zeroamplitude corresponding to no sound, the sound track in the latterregion being exposed for substantially one half of its width. Thepositive print 43 made from this negative and illustrated in Fig. 5shows at 44 the eiiect oi! the supplemental exposure. From this figureit will be seen that where the amplitude of thesound waves is a maximumthe amount of supplemental exposure is a minimum and where the soundvolume is very small or is of zero value the supplemental exposure isthe greatest. I prefer to have the two exposures slightly overlap atthose times when there is no sound on the record, however, under certainconditions it may be preferable to have them Just meet at such times oreven to come not thus leaving a narrow clear cen- Inasmuch as the filtercircuit introoertain electrical time lag in the response of the shutterthe supplemental exposing apparatus is spaced from the printer alongfilm 4 a distance which is less than the correspondin: distance alonghim 1 or the optical system This diflerence in spacing is suflicient,takmay also be varied, ii desired,

convenient, hence I arrange close as practicable to the member 29.In'makin: a variable density record with this apparaswitch 28. Such maytus, however, I prefer that the shadow shall not be sharpbut rather beas uniform as possible across the sound track. For this purpose theshutter would be arranged as close as practicable to the optical system31 and for maximum effectiveness it obviously would be adjusted to havea swing suiiicient substantially to completely cover and uncover thelight opening in the end of the optical system. Instead of the shutterconstruction which I have illustrated for varying the supplementalexposure I may in certain cases prefer to use a well known form ofoscillograph or reflecting galvanometer such as is commonly employed inthe recording of sound of the variable Width record type and as shownfor example in the Dimmick Patent 1,820,785, Aug. 25, 1931. For themaking of record copies of the variable density type I may also incertain cases prefer to use instead of the shutter apparatus illustrateda light valve such as shown for example in the Wente Patent 1,638,555,August 9, 1927, to produce the desired variation in supplementalexposure of the film copy. It is to be noted that the purpose of thefilter 27 is to iron out the waves and to provide for a relativelysmooth line of the supplemental exposure. In the event that the lightcontrolling device employed has a low natural period of vibration orperiod below the audible range, I may dispense with the filter and relyentirely upon such device to give the desired smooth line.

By providing for the elimination of ground noise at the time of making acopy of the sound record, I not only avoid unnecessarily complicatingthe recording apparatus but I am able to take any sound record withwhich there is an objectionable amount of ground noise and by making acopy thereof in accordance with my invention to greatly reduce it not toentirely eliminate the ground noise heard when the copy is used toreproduce the sound. In the event that the operator desires to renderinoperative the supplemental exposing means at any time he simply opensthe circuit by means of the be found convenient when the record includessharp or irregular sounds such as pistol shots. Ii. desired the recordfilm itself may be arranged to control the switch automatically in anywell known manner, for example, by the use of notches cut in the edge ofthe film.

I have chosen the particular embodiment described above as illustrativeof my invention and it will be apparent that various modifications maybe made without departing from the spirit and scope of my inventionwhich modiilcations I aim to cover by the appended claims.

What I claim as new and desire to secure by Letters Patent 0! the UnitedStates, is:

1. The method of making a sound record copy whichis relatively free 01'the cause oi ground noise from a previously made sound record making acontact print or the sound record, producing a current which varies inaccordance with the average amplitude of a number of successive waves ofsaid record and variably exposing the normally clear portion of theprint in accordance with said current.

2. In apparatus for making .a print 111m from ord film, means fordirecting a light beam on the print film to expose the same and meansincluding a shutter responsive to said current for varying the amount ofsaid beam reaching the print film at portions thereof unexposed by saidprinting means.

3. In apparatus for making a print film from a sound record film, meansfor printing the sound record on the print film from the record film,means for producing a current variable in accordance with the averageamplitude of a number of successive recorded waves or the record film,means for directing a light beam on an unexposed part of the print film,a, shutter in the path of said beam and means responsive to a decreasein said current corresponding to a decrease in said amplitude for movingsaid shutter in a direction to exposea greater part of said print filmunexposed by said printing means.

4. The method of making a sound record copy which is relatively freefrom the cause of ground noise from a previously made sound record whichcomprises exposing a light sensitive strip in accordance with the wavesof the sound record, producing a current which varies in accordance withthe average amplitude of a number of successive waves of said record,exposing an unexposed part oi the strip to light of a uniform intensityand varying the exposed area of of the strip in accordance with saidcurrent.

5. The method of making a sound record copy which is relatively freefrom the cause 0! ground noise from a previously made sound record filmwhich comprises printing the sound record from the record film atsuccessive points thereof on current.

6. Apparatus for making a copy of a previously made sound record whichcopy is relatively free from the cause of ground noise which lightsensitive strip in accordance with the variations of said sound record,means for producing a current which varies in accordance with theaverage amplitude of a number of successive waves of said previouslymade soundrecord and means for exposing portions of said strip otherwiseunexposed in accordance with said current.

7. Apparatus for making a copy of a previously made sound record whichcopy is relatively free from the cause of ground noise which-comprisesmeans Ior progressively exposing a light sensitive strip in accordancewith the variations of said sound record, means including a lightsensitive device associated with said record for producing a currentwhich varies in accordance with the average amplitude of a number ofsuccessive waves of said sound record, a source of light of uniformintensity arranged to expose portions of said strip otherwise unexposedand a shutter responsive to said current for controlling the exposure ofthe strip by said light. 1

CHARLES D. GREEN'I'REE.

comprises means for progressively exposing a

